Reticle for use in rapid determination of average intrafield scanning distortion having transmissivity of a complementary alignment attribute being different than the transmissivity of at least one alignment attribute

ABSTRACT

A method and apparatus for determining lens distortion in a projection imaging tool are described. The techniques include exposing at least one alignment attribute onto a substrate having a recording media. A complementary alignment attribute is also exposed onto the substrate such that the complementary alignment attribute and alignment attribute form a completed alignment attribute. The exposure of the alignment attributes, or the complementary alignment attribute, or both, may be accomplished by multiple sub nominal dose exposures. Intra field distortion of the projection imaging tool is determined from measurements of the exposed completed alignment attributes. The alignment attributes and complimentary alignment attribute may be part of a reticle. The transmission of the alignment attribute may be different than the transmission of the complementary alignment attribute.

REFERENCE TO PRIORITY DOCUMENT

This application is a continuation of U.S. patent application Ser. No.11/102,382 filed on Apr. 8, 2005 now U.S. Pat. No. 7,262,398 which is acontinuation-in-part of U.S. patent application Ser. No. 10/252,021, nowU.S. Pat. No. 6,906,303 which claimed the benefit of priority of U.S.Provisional Patent Application Ser. No. 60/323,577 filed Sep. 20, 2001and are all hereby incorporated by reference.

BACKGROUND

1. Field of the Invention

The present invention relates generally to processes for semiconductormanufacturing and more particularly to characterizing and monitoring theintra-field distortions of scanning projection systems used in ULSIphotolithography.

2. Description of the Related Art

Today's lithographic processing requires ever tighter layer-to-layeroverlay tolerances to meet device performance requirements. Overlayregistration on critical layers can directly impact device performance,yield and repeatability. Typical microelectronic devices or circuits mayhave as many as 20 or more levels or pattern layers. The placement ofpatterned features on one level must match the placement ofcorresponding features on other levels—that is, they must overlap—withinan accuracy which is some fraction of the minimum feature size orcritical dimension (CD).

Overlay error is typically, although not exclusively, measured with ametrology tool appropriately called an overlay tool using severaltechniques. See Semiconductor Pattern Overlay, N. Sullivan, SPIECritical Reviews Vol. CR52, 160:188. The term overlay metrology tool oroverlay tool means any tool capable of determining the relative positionof two alignment attributes that are separated within about 2000 um(microns) of each other. The importance of overlay error, and its impacton yield, have been extensively studied and documented. See MeasuringFab Overlay Programs, R. Martin et al., SPIE Conference on Metrology,Inspection, and Process Control for Microlithography XIII, 64:71, March1999; A New Approach to Correlating Overlay and Yield, M. Preil et al.,SPIE Conference on Metrology, Inspection, and Process Control forMicrolithography XIII, 208:216, March 1999.

Lithographers have created statistical computer algorithms (for example,Klass II (See Lens Matching and Distortion Testing in a Multi-Stepper,Sub-Micron Environment, A. Yost et al., SPIE Vol. 1087, 233:244, 1989)and Monolith (See A Computer Aided Engineering Workstation forRegistration Control, E. McFadden et al., SPIE Vol. 1087, 255:266,1989)) that attempt to quantify and divide overlay error into repeatableor systematic and non-repeatable or random effects. See Matching ofMultiple Wafer Steppers for 0.35 Micron Lithography Using AdvancedOptimization Schemes, M. van den Brink et al., SPIE Vol. 1926, 188:207,1993; A Computer Aided Engineering Workstation for Registration Control,supra; Semiconductor Pattern Overlay, supra; Machine Models andRegistration, T. Zavecz, SPIE Critical Reviews Vol. CR52, 134:159. Anoverall theoretical review of overlay modeling can be found in theliterature. See Semiconductor Pattern Overlay, supra.

Overlay error is typically divided into the following two majorcategories. The first category, inter-field or grid overlay error, isconcerned with the actual position of the translation and rotation oryaw of the image field as recorded in the photoresist on a silicon waferusing an exposure tool, i.e., stepper or scanner. The second category,intra-field overlay error, is the positional offset of an individualpoint inside a field referenced to the nominal center of an individualexposure field. Intra-field overlay errors are generally composed oflens aberrations or distortions, scanning irregularities, and reticlealignment

It is important for this discussion to realize that most overlaymeasurements are made on silicon product wafers after eachphotolithographic process, prior to final etch. Product wafers cannot beetched until the photoresist target patterns are properly aligned to theunderlying target patterns. See Super Sparse Overlay Sampling Plans: AnEvaluation of Methods and Algorithms for Optimizing Overlay QualityControl and Metrology Tool Throughput, J. Pellegrini, SPIE Vol. 3677,72:82. Manufacturing facilities rely heavily on exposure tool alignmentand calibration procedures to help insure that the scanner tools arealigning properly. See Stepper Matching for Optimum Line Performance, T.Dooly et al., SPIE Vol. 3051, 426:432, 1997; Mix-and-Match: A NecessaryChoice, R. DeJule, Semiconductor International, 66:76, February 2000;Matching Performance for Multiple Wafer Steppers Using an AdvancedMetrology Procedure, M. Van den Brink, et al., SPIE Vol. 921, 180:197,1988. Inaccurate overlay modeling algorithms can corrupt the exposuretool calibration procedures and degrade the alignment accuracy of theexposure tool system. See Super Sparse Overlay Sampling Plans: AnEvaluation of Methods and Algorithms for Optimizing Overlay QualityControl and Metrology Tool Throughput, supra.

Over the past 30 years the microelectronics industry has experienceddramatic rapid decreases in critical dimension by constantly improvingphotolithographic imaging systems. Today, these photolithographicsystems are pushed to performance limits. As the critical dimensions ofsemiconductor devices approach 50 nm the overlay error requirements willsoon approach atomic dimensions. See Life Beyond Mix-and-Match:Controlling Sub-0.18 Micron Overlay Errors, T. Zavecz, SemiconductorInternational, July 2000. To meet the needs of next generation devicespecifications new overlay methodologies will need to be developed. Inparticular, overlay methodologies that can accurately separate outsystematic and random effects and break them into assignable causes willgreatly improve device process yields. See A New Approach to CorrelatingOverlay and Yield, supra. In particular, those new overlay methodologiesthat can be implemented into advanced process control or automatedcontrol loops will be most important. See Comparisons of Six DifferentIntra-Field Control Paradigms in an Advanced Mix and Match Environment,J. Pellegrini, SPIE Vol. 3050, 398:406, 1997; Characterizing OverlayRegistration of Concentric 5× and 1× Stepper Exposure Fields UsingInter-Field Data, F. Goodwin et al., SPIE Vol. 3050, 407:417, 1997.Finally, another area where quantifying lens distortion error is ofvital concern is in the production of photo masks or reticles during theelectron beam manufacturing process. See Handbook of Microlithographyand Microfabrication, P. Rai-Choudhury, Vol. 1, 417 1997.

Semiconductor manufacturing facilities use some version of the followingcomplex overlay procedure to help determine the magnitude of intra-fielddistortion independent of other sources of systematic overlay error—infact, the technique is used for both photolithographic steppers andscanners. The technique has been simplified for illustration. SeeAnalysis of Image Field Placement Deviations of a 5× MicrolithographicReduction Lens, D. MacMillen et al., SPIE Vol. 334, 78:89, 1982. FIG. 33shows a typical overlay target—one large or outer box and one small orinner target box. FIG. 31 shows a typical portion of a distortion testreticle used in the prior art. It should be noted that the chrome targetpatterns on most reticles are 4 or 5 times larger as compared with thepatterns they produce at the image plane, this simply means modern stepand scan systems (scanners) are reduction imaging systems. Further, forpurposes of discussion, it is assumed that the reticle pattern isgeometrically perfect, (in practice, the absolute positions of featureson the reticle can be measured and the resulting errors subtracted off).First, a wafer covered with photoresist is loaded onto the wafer stageand globally aligned. Next, the full-field image of the reticle, seeFIG. 2, is exposed onto the photoresist-coated wafer. See FIGS. 31 and32. For purposes of illustration, it is assumed that the distortion testreticle consists of a 5×5 array of outer boxes evenly spaced a distanceM*P, across the reticle surface, see FIG. 2. It is typically assumedthat the center of the optical system is virtually aberration free. SeeAnalysis of Image Field Placement Deviations of a 5× MicrolithographicReduction Lens, supra. With this assumption, the reticle, shown in FIG.2 is now partially covered using the virtual reticle blades, as shown inFIG. 18, in such a way that only a single target at the center of thereticle field, box A in FIG. 2, is available for exposure. Next, thewafer stage is moved in such a way as to align the center of the reticlepattern directly over the upper left hand corner of the printed 5×5outer box array, wafer position 1 in FIG. 31. The scanner then exposesthe image of the small target box onto the photoresist coated wafer. Ifthe wafer stage, optical system and scanning dynamics were truly perfectthen the image of the small target box would fit perfectly inside theimage of the larger target box, see FIG. 33, from the previous exposure.At this point the scanner and wafer stage are programmed to step andexpose the small target box in the 5×5 array where each exposure isseparated from the previous one by the stepping distance P.

With the assumption of a perfect stage, the final coordinates of thesmall target boxes are assumed to form a perfect grid, where the spacingof the grid is equal to the programmed stepping distance, P. Finally, ifthe first full-field exposure truly formed a perfect image, then theentire 5×5 array of smaller target boxes would fit perfectly inside the5×5 array of larger target boxes. Since the first full-field exposurepattern is in fact distorted due to an imperfect imaging system (andscanner system) the actual position of the larger target box will bedisplaced relative to the smaller target boxes. The wafer is then sentthrough the final few steps of the lithographic process to create thefinal photoresist patterned overlay targets.

The resulting overlay error at each field position can be measured witha standard optical overlay tool and the result is interpreted as beingintra-field error. Using the models described below in Equations 1 and2, the overlay data can be analyzed and the lens distortion error iscalculated.

The following intra-field modeling equations are commonly used to fitthe overlay data using a least square regression technique. See Analysisof Image Field Placement Deviations of a 5× Microlithographic ReductionLens, supra; Super Sparse Overlay Sampling Plans: An Evaluation ofMethods and Algorithms for Optimizing Overlay Quality Control andMetrology Tool Throughput, supra.dxf(xf,yf)=Tx+s*xf−q*yf+t1*xf ² +t2*xf*yf−E*(xf ³ +xf*yf ²)  eq.) 1dyf(xf,yf)=Ty+s*yf+q*xf+t2*yf ² +t1*xf*yf−E*(yf ³ +yf*xf ²)  eq.)2where;(xf,yf)=intra-field coordinates(dxf,dyf)(xf,yf)=intra-field distortion at position (xf, yf)(Tx, Ty)=(x,y) intra-field translations=intra-field overall scale or magnificationq=intra-field rotation(t1, t2)=intra-field trapezoid errorE=intra-field lens distortion.

A problem with this technique is two-fold, first, it is standardpractice to assume that the wafer stage error is very small, randomlydistributed, and can be completely accounted for using a statisticalmodel. See Analysis of Image Field Placement Deviations of a 5×Microlithographic Reduction Lens, supra; A “Golden Standard” WaferDesign for Optical Stepper Characterization”, K. Kenp et al., SPIE Vol.1464, 260:266, 1991; Matching Management of Multiple Wafer SteppersUsing a Stable Standard and a Matching Simulator, M. Van den Brink etal., SPIE Vol. 1087, 218:232, 1989; Matching Performance for MultipleWafer Steppers Using an Advanced Metrology Procedure, supra. In general,positional uncertainties in the wafer stage introduces both systematicand random errors, and since the intra-field distortion is measured onlyin reference to the lithography tool's wafer stage, machine to machinewafer stage differences show up as inaccurate intra-field distortionmaps. Secondly, the assumption that lens distortion is zero at thecenter of the lens is incorrect. Furthermore, the model represented byEquations 1 and 2 is entirely unsuited to modeling scanner overlayerror—typically the intra-field distortion model accounts only forscanner skew and scanner scale overlay errors—in general, thesynchronization errors between the reticle stage and wafer stageintroduce more complex errors described below.

A technique for stage and ‘artifact’ self-calibration is described inSee Self-Calibration in two-Dimensions: The Experiment, M. Takac et al.,SPIE Vol. 2725, 130:146, 1996; Error Estimation for Lattice Methods ofStage Self-Calibration, M. Raugh, SPIE Vol. 3050, 614:625, 1997. Itconsists of placing a plate (artifact) with a rectangular array ofmeasurable targets on a stage and measuring the absolute positions ofthe targets using a tool stage and the tool's image acquisition oralignment system. This measurement process is repeated by reinsertingthe artifact on the stage but shifted by one target spacing in theX-direction, then repeated again with the artifact inserted on the stageshifted by one target spacing in the Y-direction. Finally, the artifactis inserted at 90-degrees relative to its initial orientation and thetarget positions measured. The resulting tool measurements are a set of(x, y) absolute positions in the tool's nominal coordinate system. Then,the absolute positions of both targets on the artifact and a mixture ofthe repeatable and non-repeatable parts of the stage x, y grid error arethen determined to within a global translation (Txg, Tyg), rotation (qg)and overall scale ((sxg+syg)/2) factor.

This technique has several drawbacks, including that it requires thatthe measurements be performed on the same machine that is being assessedby this technique. Furthermore, this technique requires measurementsmade on a tool in absolute coordinates; the metrology tool measures theabsolute position of the printed targets relative to its own nominalcenter; so absolute measurements are required over the entire imagingfield, with a typical size greater than about 100 mm²).

Another technique for the determination of intra-field distortion is themethod of Smith, McArthur, and Hunter (U.S. Pat. No. 6,573,986). It is aself-referencing technique that can be utilized with overlay metrologytools in a production environment. For diagnosing the intra-fieldscanner distortion in the presence of significant scannernon-repeatability, this technique teaches the use of a special reticlethat has reduced optical transmission that is multiply scanned producingsub-Eo exposures on the wafer. The result is that this technique can beused to accurately determine the repeatable part of the scannerintra-field distortion but not that part of the intra-field distortionthat changes from scan to scan, a simple example of which is the scannery-magnification.

Another drawback to these techniques to determine intra-field error isthat they use the scanner itself as the metrology tool. Due to the costof scanners, which can exceed 10 million dollars, it is desirable tohave a technique for intra-field error that does not use the scanneritself as the metrology tool for determining intra-field distortion bututilizes relatively inexpensive overlay metrology tools. Furthermore, itis desirable that the technique be easy to perform thereby allowing itto be used in a production environment by the day-to-day operatingpersonnel. It is further desirable to have a technique that measures thenon-repeatable parts of the scanner intra-field distortion.

Therefore there is a need for an effective, and efficient, way todetermine the scanner intra-field error.

SUMMARY

In accordance with the invention, techniques for determining lensdistortion in a projection imaging tool are described. The techniquesinclude exposing at least one alignment attribute onto a substratehaving a recording media. A complementary alignment attribute is alsoexposed onto the substrate such that the complementary alignmentattribute and alignment attribute form a completed alignment attribute.The exposure of the alignment attributes, or the complementary alignmentattributes, or both, may be accomplished by multiple sub nominalexposures. The completed alignment attributes will also be referred toas overlay targets.

An aperture in the projection imaging tool may be adjusted such that theexposure of the alignment attributes covers an entire field of view ofthe tool. The aperture can also be adjusted so that only thecomplementary attribute will be exposed. An intra field distortion isreconstructed from measurements of relative positions of the exposedalignment attribute and complementary alignment attribute.

The alignment attributes and complementary alignment attributes may beprovided as part of a reticle. The transmissivity of the alignmentattribute may be different than the transmission of the complementaryalignment attribute. For example, the transmission of the alignmentattribute may be greater than the transmission of the complementaryalignment attribute. Alternatively, the transmissivity of the alignmentattribute can be less than the transmissivity of the complementaryalignment attribute. Also, the transmissivity of some of the alignmentattributes may be different than the transmissivity of others of thealignment attributes. If the attributes are provided as part of areticle, then the difference in transmissivity of different portions ofthe reticle may be accomplished in various ways. For example,differential transmission can be achieved by use of phase shift maskmaterial, reflective material, or anti-reflective material.

The substrate that the attributes are exposed onto can include a varietyof materials, such as a semiconductor wafer, a flat panel display, areticle, or an electronic recording media. The recording media istypically a positive photoresist material, a negative photoresistmaterial, an electronic CCD, a diode array, a liquid crystal, or anoptically sensitive material. Also, the projection imaging tool may be aphotolithograph step and scan machine, a photolithographic scannermachine, a scanning electron beam imaging system, a scanning directwrite tool, a scalpel tool, a scanning extreme ultra-violetphotolithographic tool, or a scanning x-ray imaging system.

The techniques can be used to improve semiconductor fabrication thatuses a photolithographic projection tool. For example, a reticle with atleast one alignment attribute and a complementary alignment attributecan be provided. The transmission of the alignment attribute may bedifferent than the transmission of the complementary alignmentattribute. The alignment attribute and complementary alignment attributemay be exposed onto a substrate having a recording media such that thecomplementary alignment attribute and alignment attribute form acompleted alignment attribute. The exposure of the complementaryalignment attribute may be accomplished by multiple sub nominalexposures. Intra field distortion of the projection imaging tool may bedetermined from measurements of the exposed alignment attribute andcomplementary alignment attributes.

The techniques overcome some of the drawbacks of stage metered lensdistortion determination. For example, determining lens distortion withstage metered techniques generally assume that the stage moves nearlyperfectly, thus these techniques inherently include effects of the waferstage grid and yaw error due to stepping the reference pattern acrossthe full field exposure. Typically, the non-repeatable parts of waferstage grid and yaw error can be reduced by averaging over multiple testsof this type. In the case of a scanner, there is inherent intra-fieldvariability due to scanning synchronization error varying on a scan byscan basis so in this case, it usually requires averaging over even morefields to average out both the stepped reference pattern and thescan-to-scan intra-field variability to extract average scan behavior.To improve performance, multiple exposure techniques and/or reducedtransmission reticles are described so as to average away measurementnoise for stage metered methodologies.

Other features and advantages of the present invention should beapparent from the following description of the preferred embodiment,which illustrates, by way of example, principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a scanner exposure field, scanner slit and scannercoordinate system.

FIG. 2 schematizes a reticle used for stage metered scan and lensdistortion.

FIG. 3 shows typical overlay patterns or completed alignment attributes.

FIG. 4 shows vector plots or lens distortion error in the absence ofscanner synchronization error.

FIG. 5 shows the components making up the instantaneous scanningsynchronization error.

FIG. 6 is a schematic of the reticle for the preferred embodiment forextracting dynamic intra-field scanning error.

FIG. 7 is an exemplary overlay group, OG, for a dark field mask withdimensions in microns.

FIG. 8 is a completed alignment attribute using AA and AA′ of FIG. 7.

FIG. 9 shows on the left exemplary overlay group OG for a bright fieldmask and on the right overlay group OG as projected onto the wafer at4:1 reduction.

FIG. 10 shows a completed alignment attribute using AA and AA′ of FIG. 9as printed in positive photoresist.

FIG. 11 is a side view of the reticle of FIG. 6.

FIG. 12 shows the process flow for the preferred embodiment of thisinvention.

FIG. 13 shows a second embodiment of the preferred reticle.

FIG. 14 shows a data file that represents the final results of themethod of this invention.

FIG. 15 shows an exposure plan for determining dynamic scan error offield F.

FIG. 16 shows a wafer with wafer alignment marks suitable for using thewafer at 0 and 90 degree orientations.

FIG. 17 shows a wafer after the first exposure for determining thedynamic scan distortion.

FIG. 18 shows a wafer after exposures done at 0 degrees and 90 degrees.

FIG. 19 shows the intrafield coordinate convention.

FIG. 20 shows schematics used in FIG. 18.

FIG. 21 shows an example of a minimal overlay group as realized on adark field reticle for carrying out the method of this invention.

FIG. 22 shows overlapped overlay groups OLAP1, OLAP2, OLAP3 as realizedwith the overlay group of FIG. 21.

FIG. 23 shows an overlay group OG consisting of a pair of waferalignment marks.

FIG. 24 shows overlapped overlay groups OLAP1, OLAP2, OLPA3 as realizedwith the overlay group of FIG. 23.

FIG. 25 shows an exemplary overlay group consisting of a single waferalignment mark.

FIG. 26 shows overlapped overlay groups OLAP1, OLAP2, OLAP3 as realizedwith the overlay group of FIG. 25.

FIG. 27 shows the wafer coordinate systems used in the discussion ofthis invention.

FIG. 28 shows in cross section a partially transmitting variation of thepresent invention that utilizes a partially reflecting surface as thetransmission reduction mechanism.

FIG. 29 shows in cross section a partially transmitting variation of thepresent invention that utilizes an attenuated phased shift mask on thesurface as the transmission reduction mechanism.

FIG. 30 shows in cross section another variation of the presentinvention that utilizes a reflective reticle.

FIG. 31 shows an example of a prior art lens distortion test exposurepattern.

FIG. 32 explains the schematics used in FIG. 2.

FIG. 33 shows a perfectly overlaid box in box structure.

FIG. 34 shows OLAP1, OLAP2, and OLAP3 as realized with overlay group ofFIG. 7.

FIG. 35 is a representation of a reticle R that includes an NX×NY arrayof alignment attributes.

FIG. 36 is a flow diagram illustrating a technique for stage meteredintra-field distortion measurements.

FIG. 37 is a schematic diagram of a wafer exposed multiple times inaccordance with the process described in FIG. 36.

FIG. 38 is a flow diagram illustrating an improved technique fordetermining intra-field distortion.

FIG. 39 is a block diagram of a reticle illustrating the bladed areas ona reticle.

FIG. 40 is a flow diagram illustrating a technique where a series of subE0 exposures are used to fully expose a complementary alignmentattribute.

FIG. 41 is a flow diagram illustrating another embodiment of techniquesfor reducing noise in measurements for determining intra fielddistortion.

FIG. 42 is a flow diagram illustrating techniques for using multiple subE0 doses to expose the entire scanner field.

FIG. 43 is a flow chart illustrating another technique for reducingnoise in measurements used for determining intra field distortion.

FIG. 44 is a block diagram illustrating alignment attributes AA that areopen bars in chrome.

FIG. 45 is a block diagram illustrating an example of a complementaryalignment attribute.

FIG. 46 is a flow chart illustrating another technique for reducingnoise in measurements used for determining intra field distortion in ascanner.

FIG. 47 is a block diagram of an example of a projection imaging tool.

DETAILED DESCRIPTION

An aspect of the invention is that it does not require that measurementsbe made on the same machine that is being assessed accordinglydetermining the intra-field lens distortion can, and preferably are,made on an overlay metrology tool quite distinct from the projectionlithography tool that we are assessing.

Another aspect of the invention is that the absolute position of theprinted targets relative to the nominal center of the metrology tool isnot required, instead relative coordinates or displacements of features(box in box structures or some other alignment attribute) are measuredwith respect to each other. Because the distances between thesealignment attributes is typically less than 2.0 mm absolute position isnot required. In the case of box in box structures these distances aretypically less than about 0.2 mm. This difference is a significant onesince absolute metrology tools such as the Leica LMS 2000, Leica IPRO(See Leica LMS IPRO Brochure), or Nikon 5I (See Measuring System XY-5i,K. Kodama et al., SPIE Vol. 2439, 144:155, 1995) typically cost inexcess of 2 million dollars and are uncommon in semiconductormanufacturing facilities (fabs) while overlay metrology tools such asthe KLA 5200, or Bio-rad Q7 typically cost 0.5 million dollars and arewidely deployed in fabs. Another drawback of this technique is that itrequires that the intra-field distortion to be repeatable from exposureto exposure, this is precluded by the scanner dynamics.

Another aspect of the invention is that it utilizes a procedure thatgreatly reduces the number of measurements required to determine theintra-field lens distortion. Furthermore, the technique allows for thedetermination of the non-repeatable part of the scanner dynamicdistortion.

The structure of scanner intra-field distortion or translational errorcan be decomposed into a lens component, dependent only on theprojection imaging objective or projection system aberrations (See FIG.4), and a scanning component, dependent only on the relative dynamics ofthe wafer and reticle scanning motion. (See FIG. 5.) The lens componentis repeatable but the scanning component contains both repeatable andnon-repeatable parts. Furthermore, the lens and scanning components havecertain functional forms that simplify the extraction of intra-fielderror. A photolithographic step and scan or scanner system produces animage, typically reduced 4× or 5×, of the reticle pattern in the surfaceof the photoresist by continuously passing exposure radiation through asmall portion of the projection optics as the reticle and wafer stagetravel in opposite directions, as shown in FIG. 18. The scanning reticlestage and scanning wafer stage move in opposite directions in acoordinated manner at two different speeds.

FIG. 1 shows an instantaneous (top down) view of a partially exposedscanner field (and coordinate system) as it might appear on aphotoresist coated silicon wafer during a scan. Lack of coordinationbetween the wafer stage and reticle stage in the absence of lensdistortion will manifest itself as translational offseterror−ΔT(x,y,ys). Where ΔT(x,y,ys) is defined to be the instantaneoustranslational offset error on the wafer at intra-field positionx,y—located inside the image of the lens slit—when the scanner is atposition (ys), See FIG. 1. The final distortion error or overlay error(ΔF(x,y) at any point actually imaged in the photoresist is then anaverage of the instantaneous errors (ΔT(x,y,ys), weighted by theintensity function of the scanning slit. If the scanner operatedperfectly (without synchronization or vibration errors) then the finaldistortion or translational error, ΔsL(x) at each field point in thephotoresist would simply be the average of the static projection lensdistortion Δd(x) weighted by the intensity function of a static scannerslit. See aberration averaging; Performance of a Step and Scan Systemfor DUV Lithography, G. de Zwart et al., SPIE Vol. 3051, 817:835, 1997.

Thus, there are two independent sources of transverse scanning error orscanning distortion; projection lens distortion error—that varies inmagnitude and direction across the scanner field (in the x direction, orperpendicular to the scanning direction) and synchronization errors thatrepresent an average of the instantaneous (repeatable andnon-repeatable) positional offsets of the wafer and reticle stage.

Because the reticle and wafer move in a coordinated manner as rigidbodies relative to one another, lack of coordination will show up asinstantaneous offset errors, (ΔTx, ΔTy)(x,y,ys). Here (ΔTx, ΔTy)(x,y,ys)is the instantaneous translational offset error of the projected imageat the wafer relative to a perfectly placed wafer is a function not onlyof the intra-field coordinate (x,y) but also of the instantaneousposition, ys, of the wafer relative to the center of the scanning slit.FIG. 1 shows the relation of the full scanner field and field centerrelative to the slot center, this relative position is ys. We areconcerned here only with transverse errors of the stage and reticle andso the instantaneous offset vector (ΔTx, ΔTy)(x,y,ys) will depend onlyon the instantaneous translational offset error (ΔX(ys), ΔY(ys)) and theinstantaneous yaw or rotational error θs(ys) as:(ΔTx, ΔTy)(x,y,ys)=(ΔX(ys)+θs(ys)*(y−ys), ΔY(ys)−θs(ys)*x)  eq. 3)

Another contributor to the instantaneous offset vector will arise fromthe static distortion contribution of the projection lens. Thus if(ΔXsl, ΔYsl)(x,y) is the static lens distortion then its contribution tothe instantaneous offset vector (ΔTx, ΔTy) will be:(ΔTx, ΔTy)(x,y,ys)=(ΔXsl, ΔYsl)(x,y−ys)  eq.3a)

The static lens distortion means the intra-field distortion of thescanner as determined when the wafer and reticle stages are not movedwith respect to one another to produce the scanned image field. Thus,the static lens distortion does not include any contribution fromsynchronization or dynamic yaw errors due to the relative motion of thereticle and wafer stages. Referring to FIG. 1, (ΔXsl, ΔYsl)(x,y) isdefined only over the slot width (SW) and slot height (SH). Therefore x,y vary over the rangesx=(−SW/2:SW/2) y=(−SH/2:SH/2)  eq.3b)

There are various techniques for determining (ΔXsl, ΔYsl), a veryaccurate technique is described in “Method And Apparatus ForSelf-Referenced Projection Lens Distortion Mapping” (U.S. Pat. No.6,573,986) but this and other techniques for measuring static lensdistortion are not required for the techniques described below.

Combining Equations 3 and 3a give the total contribution to theinstantaneous offset error as:(ΔTx, ΔTy)(x,y,ys)=(ΔXsl, ΔYsl)(x,y−ys)+(ΔX(ys)+θs(ys)*(y−ys),ΔY(ys)−θs(ys)*x)  eq. 3c)Here x,y vary over the entire span of intrafield coordinates;x=(−SW/2:SW/2) y=(−L/2:L/2)  eq.3d)while ys varies over the range:ys=(y−SH/2:y+SH/2)  eq.3e)Since the projected image suffers a shift only when the slot (or moreprecisely any part of the illuminated slot) is over field position(x,y).

The effect of the projected image is then just a weighted average overthe slot of the instantaneous offsets (ΔTx, ΔTy):(ΔXF, ΔYF)(x,y)=INT{dys*w(y−ys)*(ΔTx,ΔTy)*(x,y,ys)}/INT{dys*w(y−ys)}  eq. 3f)where;x,y=intrafield coordinates, x=(−SW/2:SW/2), y=(−L/2:L/2)ys=the position of the center of the scanning slit at a given instant intime referenced from the nominal die centerSW=slot widthL=scanner field lengthdys=differential amount of the scanner fieldINT{ }=integral over the scanner field, integration range extends fromys=(−(L+SH)/2:(L+SH)/2))w(y)=weighting function. In 248 nm resists, typically proportional tothe slot intensity profile scanning slit. 0 for points outside the slitopening.(ΔXF, ΔYF)(x,y)=intrafield distortion. Includes effects of scanningsynchronization error and lens aberrations.

The two distinct parts of (ΔTx, ΔTy) (scanner dynamics (eq. 3) and lensdistortion (eq. 3a)) are additive and therefore the intrafielddistortion, (ΔXF, ΔYF), can also be divided up into similar parts as:(ΔXF, ΔYF)(x,y)=(ΔxL, ΔyL)(x)+(ΔXS(y), ΔYS(y)−x*dΔYS(y)/dx)  eq. 3g)where the lens aberration contribution, (ΔxL, ΔyL)(x), is given by;(ΔxL, ΔyL)(x)=INT{dys*w(y−ys)*(ΔXsl,ΔYsl)(x,y−ys)}/INT{dys*w(y−ys)}  eq. 3h)and the scanning dynamics contribution, (ΔXS(y), ΔYS(y)−x*dΔYS(y)/dx),is given by;(ΔXS(y), ΔYS(y)−x*dΔYS(y)/dx)=INT{dys*w(y−ys)*(ΔX(ys)+θs(ys)*(y−ys),ΔY(ys)−θs(ys)*x)}/INT{dys*w(y−ys)}  eq. 3i)

Identifying separate components in Equations 3h) and 3i) gives theindividual expressions for the various components of overlay error. Thusthe dynamic slip in the x and y directions due to synchronization erroris given by;ΔXS(y)=dynamic slip in the xdirection=INT{dys*w(ys)*ΔX(y−ys)}/INT{dys*w(ys)}  eq.3j)ΔYS(y)=dynamic slip in the ydirection=INT{dys*w(ys)*ΔY(y−ys)}/INT{dys*w(ys)}  eq.3k)the dynamic yaw or rotational error due to synchronization error isgiven by;dΔYS(y)/dx=dynamic yaw=INT{dys*w(ys)*θs(ys))}/INT{dys*w(ys)}  eq.31)

The influence of the dynamic lens distortions on the intra-field error,(ΔxL, ΔyL), is given by;ΔxL(y)=dynamic lens distortion in the xdirection=INT{dys*w(ys)*ΔXsl(y−ys)}/INT{dys*w(ys)}  eq.3m)ΔyL(y)=dynamic lens distortion in the ydirection=INT{dys*w(ys)*ΔYsl(y−ys)}/INT{dys*w(ys)}  eq.3n)

The interpretation of the structure of the intra-field distortion, (ΔXF,ΔYF), is best understood with reference to Equation 3g). There, theintra-field distortion is divided into a contribution by the dynamiclens distortion, (ΔxL, ΔyL), that depends only on the cross scancoordinate, x, and is independent of the position along the scanningdirection, y. From equations 3m) and 3n), the dynamic lens distortion isa weighted average of the static lens distortion where the weightingfactor, w(y), depends on the intensity distribution in the scandirection, y, possibly the photoresist process, and the scanningdirection. Because the dynamic lens distortion contains none of theeffects of scanning synchronization errors and only effects that arehighly repeatable, the dynamic lens distortion will not vary from scanto scan. Thus, the contribution of dynamic lens distortion to theintrafield distortion can be some arbitrary set of vector displacementsalong a single scan row but will be the same for all rows in the scan,see FIG. 4.

The other contributor to intra-field distortion in Equation 3g) is thedynamic slip and yaw errors, ΔXS(y), ΔYS(y), dΔYS(y)/dx, which depend onthe position along the scanning direction, y, and are independent of thecross scan coordinate, x. From Equations 3j), 3k), 3l) the dynamic slipand yaw are convolutions of the weighting factor w(y) with theinstantaneous translational and yaw offsets. Because dynamic slip andyaw contain nothing but the effects of scanner synchronization error,they will contain both repeatable parts that do not vary from scan toscan and non-repeatable parts that vary from scan to scan. Referring toFIG. 5, each row of the scan will have different translation androtation errors that are generally different and strongly correlatedonly over distances less than about SH, the slot height.

In summary, in the presence of both lens distortion and scannersynchronization error the total overlay distortion error, [δX(x,y),δY(x,y)] can be expressed in the following form;δX(x,y)=ΔXS(y)+ΔxL(x),  eq.12)δY(x,y)=ΔYS(y)+ΔyL(x)−x*dΔYS(y)/dx  eq.13)

In acid catalyzed photoresists such as those used for KrF or 248 nmlithography, the weighting function will typically be directlyproportional to the intensity of light, I(y), across the slot since thelatent acid image does not saturate until at very high exposure doses.However, in typical I-line photoresists the latent image saturates atnormal exposure doses. This means that at a given location on thephotoresist, the exposing light that first impinges consumes a largerportion of the photoactive material than an equal amount of exposinglight impinging at a later time. Thus the w(y) will not be proportionalto I(y) any longer. Because of this saturation effect, the weightingfunction will depend not only on the photoresist exposure dose used butalso on the scanning direction (positive y or negative y).

First Embodiment

A method for determining the distortion associated with scannersynchronization error (scan error for short) to within a translation,rotation, and skew in the presence of scanner lens distortion isdescribed. The process flow for the first embodiment is diagramed inFIG. 12.

Provide Reticle

Referring to FIG. 6, a reticle, OL, with an (Mx×My) array of overlaygroups, OG, is provided, loaded into a projection lithography tool(machine) being measuring, and aligned to reticle alignment mark RM.Reticle OL, shown in cross section in FIG. 11, may be a glass or fusedsilica reticle with a chrome coating that defines the overlay groups,OG; it is a binary mask. FIGS. 7 and 9 show realizations of OG for thefirst embodiment. They both consist of alignment attributes, AA, andcomplementary alignment attributes, AA′, offset from AA a distance M*dp.When overlaid one on top of another, AA and AA′ form completed alignmentattributes, CAA, illustrated in FIGS. 8 and 10. FIG. 8 is the completedalignment attribute, CAA, as viewed on the wafer consisting of theprojection of alignment attribute AA and complementary alignmentattribute AA′ of FIG. 7 on top of one another. The inner square torus ofFIG. 8 represents the projection of AA′ while the outer square torusrepresents the projection of AA onto the wafer. The darkened areasrepresent exposed photoresist or other recording media.

FIG. 7 is a realization of overlay group OG for a dark field mask. Thedarkened areas represent chrome removed from the reticle and typicaldimensions in microns are shown. These dimensions are appropriate whenoverlay reticle OL is used in a 4:1 (M=4 in FIG. 6) or 5:1 (M=5 in FIG.6) reduction imaging tool. When used in a 1:1 imaging tool (nomagnification or demagnification of the image size) the dimensions shownin FIG. 7 would be reduced by approximately 4-5 times so that thecompleted alignment attributes (CAA of FIG. 8) would be within therecommended size range for bar in bar structures suitable for an overlaymetrology tool, typically about 15-30 um. See Overlay Target Design,KLA-Tencor, KLA-Tencor, 1:4, 1996. M*dp of FIG. 6 is the distancebetween alignment attributes AA and their complements AA′ and for theexample of FIG. 7 is equal to 500 microns.

FIG. 9 is another realization of overlay group OG, this time for abright field mask. The darkened areas represent chrome remaining on thereticle and typical dimensions in microns are shown. These dimensionsare appropriate when overlay reticle OL is used in a 4:1 (M=4 in FIG. 6)or 5:1 (M=5 in FIG. 6) reduction imaging tool. The same comments aboveapply to the design basis for this size and adaptation to imaging toolswith other magnifications, M. M*dp of FIG. 6 is the distance betweenalignment attributes AA and their complements AA′ and for the example ofFIG. 9 is equal to 500 microns. FIG. 10 is the completed alignmentattribute, CAA, as viewed on the wafer consisting of the projection ofalignment attribute AA and complementary alignment attribute AA′ of FIG.9 on top of one another. The inner square box of FIG. 8 represents theprojection of AA′ while the outer square box represents the projectionof AA onto the wafer. The darkened areas represent resist remaining onthe wafer, in the case of a positive tone resist.

Referring to FIG. 6, overlay groups OG are separated a distance M*p″where p″ is typically in the range of 0.5 mm to 10 mm when used onsemiconductor wafers. M is the reduction magnification ratio of theprojection imaging tool used. For semiconductor manufacturing this istypically M=1, 4 or 5, most commonly 4 or 5. Thus an exemplary dimensionfor M*p″ for an M=4 or M=5 system is 4 mm leading to a pitch, p″, of theprojected pattern on the wafer of p″=1 mm (M=4) or p″=0.8 mm (M=5).Typical values for p″ are in the range of 0.5 mm to 10 mm while typicalvalues for dp are 0.02 mm to 1 mm. The significant constraint on p″ isthat it be small enough to provide detailed enough coverage of the scandistortion pattern. Stated differently, we need to sample the scandistortion at a fine enough interval such that the distortions at theunmeasured locations in between the overlay groups are reasonablyapproximated (error less than 30% maximum distortion) by interpolatingthe values of scan distortion measured on pitch p″. The significantconstraint on offset dp is that it lie within an area where the scandistortion is not varying significantly. Stated differently, the overlaygroup of FIG. 6 should lie within an isoplanatic distortion patch of thescan field, herein defined as being a region over which the scannerdistortion varies by <5% of the maximum value of the scan distortion.

Also disposed on overlay reticle OL will be reticle alignment marks, RM,that allow the reticle to be precisely aligned with respect to theprojection imaging tool it is used on.

The number of overlay groups OG on reticle OL is determined by themaximum projected field size of the machine or set of machines we willbe measuring. In cases where the extent of the overlay groups on thereticle exceeds the size of the maximum field, the entire Mx×My array isnot required, a smaller section that fits within the maximum field orother user designated field will work with the method of this invention.

Load/Align Reticle

Next, overlay reticle OL is loaded into the projection lithography tool(machine) and aligned. The reticle alignment is typically carried outusing reticle alignment marks, RM. On lower accuracy machines, largeralignment attributes AA and their complements, AA′, when combined withmechanical banking or placement of the reticle may suffice for reticlealignment. In these circumstances, no reticle alignment marks would berequired.

Provide/Load/Align Wafer

Next, a photoresist coated wafer is provided. Referring to FIG. 16, thiswafer may have already disposed on it global wafer alignment marks GM0and GM90. GM0 is the wafer alignment mark suitable for the wafer when itis loaded with the notch in the default or 0 degree orientation. Twomarks, shown in FIG. 16, and possibly more, are typically required forwafer alignment. The required alignment accuracy for semiconductorwafers and standard box in box or bar in bar completed alignmentattributes will typically be less than about 2 um. This is so theoverlay tool metrology used for measuring the completed alignmentattributes is operating in the regime where it is most accurate andrepeatable. See KLA 5105 Overlay Brochure, KLA-Tencor. GM90 is thealignment mark suitable for the wafer when loaded with the notch in therotated 90 degrees from the default or 0 degree orientation. Two marksare shown in FIG. 16. In cases where the wafer prealignment system canmeet the required tolerances by aligning off of the wafer edge andnotch, an unpatterned wafer can be used. Once provided, the wafer isthen loaded and aligned on the projection lithography tool we aremeasuring.

Expose Reticle

Next, referring to FIG. 17, overlay reticle OL is exposed projecting anNx×Ny array of overlay groups, OG, from reticle OL onto wafer Wresulting in an Nx×Ny array of projected overlay groups, POG, on waferW. The entire projected array comprises a field F over which we will bemeasuring the machine dynamic scan distortion; the present inventionwill determine the synchronization or dynamic distortion present in thissingle realization of scanning distortion as present in the field F.

Rotate/Align Wafer

Following the first exposure the wafer is rotated by 90 degrees andrealigned using global wafer alignment marks GM90. For the rotationstep, the wafer may have to pass out through the track, skipping theresist development cycle and be passed back through track, skipping theresist coating cycle, and reinserted onto the wafer chuck. In somecases, the wafer may need to be rotated by hand approximately 90 degreesbefore the machine prealignment system can accommodate it. In any event,once the wafer has been rotated, it is then aligned as discussed aboveonly the GM90 marks are utilized. In this case the global waferalignment marks GM0 remain individually identical in appearance oncethey have been rotated by 90 degrees, then in their new position theycan serve the same function as marks GM90. For the purposes of thisinvention the wafer can be rotated either clockwise or counterclockwiseby 90 degrees. The description of the preferred embodiment assumes thewafer is rotated clockwise by 90 degrees as indicated by FIG. 27.

Expose OL Reticle to Create Completed Alignment Attributes

Next the wafer is exposed with the overlay reticle OL one or more timesresulting in an Nx×Ny array of projected overlapped overlay groupsconsisting of one or more of the following types, OLAP1, OLAP2 or OLAP3,See FIGS. 18 and 34). Referring to FIG. 15, field F is shown as a dashedrectangle longer than it is wide and with the scanning directionindicated by an arrow. This is typical of the dimensions of a scannedfield since the purpose of the scanning mechanism is to enlarge theprojected imaging field by utilizing the mechanical synchronization ofthe wafer and reticle stage and thereby minimize the area projected byimaging objective. See Optical Lithography—Thirty Years and Three Ordersof Magnitude, J. Bruning, SPIE Vol. 3051, 14:27, 1997. Typical maximumscanned field dimensions for semiconductor wafer scanners are 22×32.5,25×33, 26×33, and 26×34 (SW×L in mm per FIG. 1). Thus for semiconductorwafer scanners, to create completed alignment attributes at all Nx×Nyprojected overlay groups two separate scans (with fields R1 and R2 inFIG. 15) are required. While the fields R1 and R2 could be done withoutany overlay region, OL, the resulting measurement set of completedalignment attributes could only partially determine the dynamic scanerror over the entire field. While this can be useful in the case whereonly a small portion of the entire scanned field is to be analyzed orthe projected field of interest is small enough that only a singlefield, R1, will overlay the field of interest, F, the overlapping2-field case represents the preferred embodiment. Cases where fewer ormore fields are required to overlap F are easily adapted from thisembodiment. FIG. 20 explicitly shows which overlay groups in FIG. 18result from which exposure (first, R1, R2).

When viewed with the notch at nominal or 0 degree orientation, (See FIG.18), exposure R1 of reticle OL of FIG. 6 consists of an Nx×Ny′ array ofoverlay groups (dashed lines of FIG. 18) placed to form completedalignment attributes, CAAL, when combined with the overlay groupsdefined by the field F exposure (solid lines of FIG. 18). Ny′ is lessthan Ny allowing R1 to be placed so. If the field F has been exposedwith center located at wafer coordinates (xc,yc) then for the reticlelayout of FIG. 6, then the center of exposure R1 will be made atexposure coordinates (FIG. 27) (xe,ye)=(yc−p″*(Ny−Ny′)/2−dp, −xc).Exposure coordinates (xe,ye) do not rotate with the wafer but coincidewith the wafer coordinates when the wafer has its notch at the 0 degreeor nominal orientation. Both wafer and exposure coordinates have thewafer center, WC of FIG. 27, as their origin. So, having completed theR1 exposure, there results an Nx×Ny′ array of completed alignmentattributes for the lower portion of the field F (CAAL of FIG. 18).

Next, exposure R2 is made covering the upper portion of field F andconsisting of an Nx×Ny″ array of overlay groups (dash dot lines of FIG.18) that are placed to form an Nx×Ny″ array of completed alignmentattributes, CAAU, for the upper portion of field F. Ny″ is less than Nyso the CAAU array extends from row b=Ny−Ny″+1 to b=Ny. So that we candiagnose the scan distortion over the entire field F, the lower andupper exposures, R1 and R2, need to overlap at least two overlay groups.Referring to FIG. 18, rows b=Ny−Ny″+1 through b=Ny′ will consist ofprojected overlapped overlay groups OLAP2 each of which consists ofcompleted alignment attributes CAAL and CAAU. In terms of Ny′ and Ny″this means Ny′+Ny″>=Ny+2. With the field F placed at (xc,yc), exposureR2 will be placed at exposure coordinate (xe,ye)=(yc+p″*(Ny−Ny″)/2+dp,−xc) resulting in the dash dot overlay groups of FIG. 18.

The net result of exposures F, R1 and R2 is to create an Nx×Ny−Ny″ arrayof projected overlapped overlay groups, OLAP1, each containing at leastone completed alignment attribute, CAAL, of fields F and R1. Further, anNx×Ny′−Ny+Ny″ array of projected overlapped overlay groups, OLAP2, eachcontaining at least one completed alignment attribute, CAAL, of fields Fand R1 and at least one completed alignment attribute, CAAU, of fields Fand R2. Further, an Nx×Ny−Ny′+1 array of projected overlapped overlaygroups, OLAP3, each containing at least one completed alignmentattribute, CAAU, of fields F and R2.

Develop Wafer

The wafer is then developed.

Measure Overlay Targets

Next, an overlay metrology tool is used to determine the positionaloffset error of at least 2 columns of completed alignment attributes.Thus, in the first embodiment, the two outer columns, a=1 and a=Nx ofFIG. 18 would be measured. Within each measured column, all completedalignment attributes, Ny′ CAAL and Ny″ CAAU, for a total of Ny′+Ny″would be measured. The effect of not measuring an alignment attributeCAAL or CAAU is that we lose information concerning scanner distortionfor that particular row, however we need to measure at least 2 rowswhere the alignment attributes lie within OLAP2 groups.

Provide Lens Distortion Map

Next, a map of the dynamic lens distortion for the machine beingmeasured is provided. The dynamic lens distortion (eq. 3a) representsthe effect of lens aberrations on intrafield distortion. Lens distortionis constant over short time periods (less than about one day) andtherefore its contribution can be determined in advance and used forcorrections and improvements in accuracy for the present determinationof scanning distortion.

There are numerous methods for determining dynamic lens distortion themost accurate of which is the method of Smith, (U.S. Pat. No.6,906,780). Another technique for the determination of lens distortionis the method of Smith, McArthur, and Hunter (U.S. Pat. No. 6,573,986).This technique can be applied to measure the repeatable part of thescanner distortion along with the lens distortion, the resulting2-dimensional field fit to the functional form for scanner intra-fielddistortion (eq. 3g) and the dynamic lens distortion extracted. Yetanother technique involves exposing a dynamic field a single time andmeasuring the absolute positions of the printed features using anabsolute position metrology tool such as the LMS IPRO. See Leica LMSIPRO Brochure, supra. Again, the resulting 2-dimensional field fit tothe functional form for scanner intra-field distortion (eq. 3g) and thedynamic lens distortion extracted.

In cases where the scanning distortion is large compared to the lensdistortion, the contribution from lens distortion can be neglected.

Reconstruct Scanner Distortion Map

At this point, a software algorithm is used to calculate the scannerdistortion the result being a table, as shown in FIG. 14, consisting ofthe scanning distortion as a function of the scan (y) position. Whatfollows are details of the software algorithm.

As noted above, Equations 12 and 13 show that the intrafield distortionerror in the presence of scanner synchronization error and lensdistortion is the sum of two vector parts;δX(x,y)=ΔXS(y)+ΔxL(x),  eq.12)δY(x,y)=ΔYS(y)+ΔyL(x)−ΔYR(x,y)  eq.13)

Where (x, y) are the intrafield coordinates. They are centered on fieldF and shown in FIG. 19. Also, ΔXS(y), ΔYS(y), represent the integratedaverage translational error associated with the scanning dynamics,ΔxL(x), ΔyL(x), represent the translational error associated with lensdistortion and ΔYR(x,y) represents the integrated scanning average Yawerror (ΔYR(x,y)=x*[dΔYS(y)/dx]=x*[θavg(y)]).

The deviation of the overlay groups in field F from their idealpositions (dxF,dyF)(x,y) is given by:dxF(x,y)=Tx−q*y+ΔxL(x)+ΔXS(y)  eq. 14)dyF(x,y)=Ty+q*x+ΔyL(x)+ΔYS(y)+x*θavg(y)  eq. 15)where Tx, Ty, q represent a gross intrafield translation and rotationdue to reticle and stage mispositioning.

The deviation of the overlay groups in field R1 from their idealpositions (dxR1,dyR1)(x,y) is given by:dxR1(x,y)=Tx′−q′*y−ΔyL(y+n1*p″)+ΔYS′(x)+y*θ′avg(x)  eq. 16)dyR1(x,y)=Ty′+q′*x+ΔxL(y+n1*p″)+ΔXS′(x)  eq. 17)where n1=when field R1 is centered within the maximum allowed exposurefield and Tx′, Ty′, q′ are another set of translations and rotation.

The deviation of the overlay groups in field R2 from their idealpositions (dxR2,dyR2)(x,y) is given by:dxR2(x,y)=Tx″−q″*y−ΔyL(y−n2*p″)+ΔYS′(x)+y*θ″avg(x)  eq. 18)dyR2(x,y)=Ty″+q″*x+ΔxL(y−n2*p″)+ΔXS″(x)  eq. 19)where n2=when field R2 is centered within the maximum allowed exposurefield and Tx″, Ty″, q″ are yet another set of translations and rotation.

Denoting now the sign of the displacement for the outer box by + and thesign of the inner box by −, the lower completed alignment attributes,CAAL, produce overlay measurements:BBx(x,y;L)=Tx−Tx′+ΔxL(x)−ΔYS′(x)+(−q+q′−θ′avg(x))*y+ΔyL(y+n1*p″)+ΔXS(y)  eq.20)BBy(x,y;L)=Ty−Ty′+ΔyL(x)−ΔXS′(x)+(q−q′+θavg(y))*x−ΔxL(y+n1*p″)+ΔYS(y)  eq.21)while the upper completed alignment attributes, CAAU, produce overlaymeasurements:BBx(x,y;U)=Tx−Tx″+ΔxL(x)−ΔYS″(x)+(−q+q″−θ″avg(x))*y+ΔyL(y−n2*p″)+ΔXS(y)  eq.22)BBy(x,y;U)=Ty−Ty″+ΔyL(x)−ΔXS″(x)+(q−q″+θavg(y))*x−ΔxL(y−n2*p″)+ΔYS(y)  eq.23).

In the region where R1 and R2 overlap the projected overlay groups,OLAP2, contain both an upper, CAAU, and lower, CAAL, completed alignmentattribute. The difference between the upper and lower overlaymeasurements at the same position and putting the known lens distortionson the left hand side gives:BBx(x,y;U)−BBx(x,y;L)−ΔyL(y−n2*p″)−ΔyL(y+n1*p″)=Tx″+Tx′−ΔYS″(x)+ΔYS′(x)+(q″−q′−θ″avg(x)+θ′avg(x))*y  eq.24)BBy(x,y;U)−BBy(x,y;L)−ΔyL(y−n2*p″)−ΔyL(y+n1*p″)=−Ty″+Tx′−ΔXS″(x)+ΔXS′(x)+(−q″+q′)*y  eq.25)The interpretation of Equations 24 and 25 is that we know what thetranslation and rotation of each column in the upper section relative tothe lower section and that therefore, by applying Equations 24 and 25 at2 or more points in y along each column, we can fix the location of thelower set of completed alignment attributes, CAAL, to the upper sectionof completed alignment attributes, CAAU.

Further interpreting Equations 20-23, considering a specific column orfixed x value, since the uncertainty or unknown part of the lensdistortion will typically consist of a translation, rotation andx-scale. Based on these unknown quantities, and utilizing data from 2distinct columns (y values) of field F, we will be able to determineΔXS(y) to within an expression of the form a+b*y, θavg(y) to within aconstant d, and ΔYS(y) to within a constant c. Taken altogether, we willbe able to determine the scanner distortion (ΔXS(y), ΔYS(y)+θavg(y)*x)to within an expression of the form (a+b*y,c+d*x) where a, b, c, d areunknown constants. In other words, we will know the scanning distortionto within a translation, rotation and skew (b term).

Equations 20-23 are typically solved using the singular valuedecomposition to produce the minimum length solution. See NumericalRecipes, The Art of Scientific Computing, W. Press et al., CambridgeUniversity Press, 52:64, 1900. They are typically over-determined in thesense of equation counting (there are more equations than unknowns) butare still singular in the mathematical sense; there is an ambiguity inthe solution of these equations. This ambiguity in the four parameterset discussed above for the wafer stage can also induce intrafieldrotation errors.

At this point we have accomplished the last step in the process of thisinvention and we can record the final results of the scanning distortionin tabular form (FIG. 14).

Second Embodiment

Instead of the reticle of FIG. 6, this invention could be carried outwith the reticle layout of FIG. 13. It too consists of an Mx×My array ofoverlay groups OG on regular pitch M*p″ the only difference being in thedetails of the overlay group. Now overlay group OG consists of alignmentattribute AA and only a single complementary alignment attribute, AA′,offset from it in a single direction. An example of an overlay groupwith this structure is shown in FIG. 21. There a dark field reticledesign consists of outer bar alignment attribute AA and thecomplementary alignment attribute consists of an inner bar alignmentattribute AA′. Reticle dimensions suitable for an M=4 or 5 reductionimaging lithography tool are shown. FIG. 22 shows how projectedoverlapped overlay groups OLAP1, OLAP2 and OLAP of FIG. 18 would appearwhen the overlay group of FIG. 21 is utilized. Lower, CAAL, and upper,CAAU, completed alignment attributes are also indicated. The dark areascorrespond to exposed resist. Other than the appearance of the overlaygroups, this reticle would be used in the same way as the reticlementioned in the preferred embodiment.

Third Embodiment

In this case, the overlay groups OG of reticle OL (FIG. 6) consist of apair of wafer alignment marks. Referring to FIG. 23, overlay group OGconsists of alignment attribute AA and offset from it is complementaryalignment attribute AA′. AA is a wafer alignment mark, WAM0, suitablefor use by a lithography tool wafer alignment system and stage when thewafer is in the nominal or 0 degree position. AA′ is a wafer alignmentmark, WAM90, which is wafer alignment mark WAM0 rotated by 90 degrees ina clockwise direction. FIG. 24 shows how projected overlapped overlaygroups OLAP1, OLAP2 and OLAP of FIG. 18 would appear when the overlaygroup of FIG. 23 is utilized. Lower, CAAL, and upper, CAAU, completedalignment attributes are also indicated. The exposure steps of thepreferred embodiment must be altered is an obvious way so the waferpattern results in the projected overlapped overlay groups OLAP1, OLAP2and OLAP of FIG. 24. The other step that differs in detail is that ofmeasuring the overlay targets. In this instance, instead of using anoptical overlay metrology tool, the lithography tool wafer stage andalignment system is used. The completed alignment attribute is a pair ofwafer alignment marks (FIG. 3 and CAAL, CAAU of FIG. 24) and thelithography system measures the offset of the two alignment marks AA′and AA and the nominal offset, (D,0), is subtracted from this resultingin the required overlay measurement. The nominal offset, (D,0), isdetermined by the details of the exposure plan and the minimumseparation requirements of the wafer alignment system. Typically, D isless than about 0.5-1 mm so that the wafer stage is utilized overextremely small distances where it's accuracy will be greatest. So, whenreferring to overlay metrology tools, we also encompass absolutepositioning metrology tools used over small (<4 mm) distances. Waferalignment mark WAM need not be the same wafer alignment mark as themachine we are measuring, it could be another absolute positioningmetrology tool. This embodiment is useful for embedding the entireprocedure and technique of this invention into a lithography tool forself-analysis.

Fourth Embodiment

In this case, the overlay groups OG of reticle OL (FIG. 6) consists of asingle wafer alignment mark. Referring to FIG. 25, overlay group OGconsists of alignment attribute AA which is complementary to itself whenrotated by 90 degrees. WAM is a wafer alignment mark suitable for use bya lithography tool wafer alignment system. FIG. 26 shows how projectedoverlapped overlay groups OLAP1, OLAP2 and OLAP of FIG. 18 would appearwhen the overlay group of FIG. 25 is utilized. Lower, CAAL, and upper,CAAU, completed alignment attributes are also indicated. The exposuresteps of the preferred embodiment must be altered is an obvious way sothe wafer pattern results in the projected overlapped overlay groupsOLAP1, OLAP2 and OLAP of FIG. 26. The detailed method and description ofmeasuring the completed alignment attributes is as described in thethird embodiment. This embodiment is extremely useful when the procedureand technique is embedded into the projection imaging tool for use inself-analysis.

Fifth Embodiment

FIG. 28 shows a fifth embodiment of this invention. When it is desiredto measure the repeatable part of the dynamic scan distortion with aminimum number of overlay measurements, the two reticles are used. Thefirst reticle, OL, is the one already described above. The secondreticle, OL′, is the reticle of FIG. 6 as modified with the addition ofa partially reflecting coating, PR, to the surface opposite thepatterned chrome surface (FIG. 28). There, partially reflecting coatingPR will typically reflect 50% to 99% of the incident light used forresist exposure while patterned chrome surface, PS, contains overlaygroups OG. Thus, overlay reticle OL′ is a reduced transmission reticle,meaning it's transmission is less than that of a normal reticle. Inoperation, the step of “Expose Reticle” which produces field F of FIG.17 is carried out with reticle OL′. Now instead of a single exposure,because of the reduced net reticle transmission (as produced bypartially reflecting coating PR), multiple exposures are made so theresist receives the correct clearing dose. The effect of doing Nexposures to make field F is that the non-repeatable part of the dynamicscan distortion is averaged over a number of exposures proportional to Nthereby reducing its contribution to the net dynamic scan distortion.After the “Expose Reticle” step has been carried out, the remainingsteps, as previously described, are carried out. In particular, the stepof “Expose OL Reticle to Create Completed Alignment Attributes” iscarried out using ordinary reticle OL.

Sixth Embodiment

FIG. 29 shows a sixth embodiment of this invention. This is anotherspecific variation of the reduced transmission reticle, OL′, of thefifth embodiment only now instead of having a partially reflectingcoating on the back side, the patterned face that contains the overlaygroups, OG, is patterned as an attenuated phase shift mask. See TheAttenuated Phase Shift Mask, B. Lin, Solid State Technology, SpecialSeries/Advanced Lithography, 35(1):43-47, (January, 1992). The overlaygroups are patterned using only the attenuated phase shifting material.It is not the phase shifting property of this layer that is significantonly its transmission characteristics which are typically less thanabout 10%. In all other respects, this embodiment is identical to thefifth embodiment.

Seventh Embodiment

FIG. 30 shows a seventh embodiment of this invention. Here instead ofreticle OL of FIG. 6 being a transmissive reticle it is a reflectivereticle. In a dark field version of this (FIG. 30) the overlay groupsare defined by the presence of a reflective layer on the mask.

Exposure Averaging Embodiments

Exposure averaging embodiments, such as using reduced transmissionreticles or other techniques for reducing the dose of exposure andcarrying out multiple exposures, can be useful in reducing the effectsof random errors in a stepped reference pattern. As noted, the stagemetered lens distortion technique is the standard industry technique fordetermining intra-field distortion (lens, or combined lens and scan).This technique inherently includes effects of the wafer stage grid andyaw error due to stepping the reference pattern across the full fieldexposure. The non-repeatable parts of wafer stage grid and yaw can bereduced by averaging over multiple tests of this type. In the case of ascanner, there is inherent intra-field variability due to scanningsynchronization error varying on a scan by scan basis so in this case,it is typical to require averaging over even more fields to average outboth the stepped reference pattern and the scan to scan intra-fieldvariability when it is desired to extract average scan behavior.

FIGS. 35, 36, and 37 illustrate aspects of the stage metered intra-fielddistortion measurement technique. FIG. 35 is a representation of areticle R that includes an NX×NY array of alignment attributes, AA. Thereticle R also includes a single complementary alignment attribute AA′toward the top of edge of FIG. 35. In the example illustrated in FIG.35, the alignment attributes AA are outer bars, or outer boxes, and thecomplementary alignment attribute AA′ is an inner bar pattern, or aninner box.

FIG. 36 is a flow diagram illustrating a technique for performing stagemetered intra-field distortion measurements. Flow begins in block 3602where a reticle is provided. Flow continues to block 3604 where a photoresist wafer is provided. Flow continues to block 3606. In block 3606the entire projection field of the projection imaging tool, or machine,is exposed onto the wafer stage with the reticle. Flow continues toblock 3608. In block 3608 image field blades are used to decrease theaperture so that only the complementary alignment attribute, AA′, willbe exposed. The wafer stage is then stepped so that the complementaryalignment attribute is exposed so that it is interlocked with thealignment attributes that were previously exposed, thereby formingcompleted alignment attributes. Exposure of the complementary alignmentattribute is repeated across the wafer stage until all, or a desirednumber, of completed alignment attributes are formed. Flow continues toblock 3610 and the wafer is developed and measurements made of thecompleted alignment attributes, or overlay targets. Flow then continuesto block 3612 and the intra-field distortion of the projection imagingtool, or machine, is reconstructed.

FIG. 37 is a schematic diagram of a wafer that has been exposed multipletimes in accordance with the process described in FIG. 36. In theexample of FIG. 37, the entire field of the reticle R has been exposedfour times resulting in four printed fields PF1-PF4. In this example,each printed field PF1-PF4 is a 3×3 array of printed alignmentattributes PAA. After the printed fields were exposed the reticle R wasapertured down so only complementary alignment attribute AA′ could beexposed and the wafer stage was stepped around the area of each printedfield nine times to create nine printed complementary alignmentattributes, PAA′. The superposition of printed alignment attributes andprinted complementary alignment attributes results in a printedcompleted alignment attribute, PCAA, which can be read with an overlayreader which measures an offset of a PAA′ from its corresponding PAA.The resulting measurements are:(BBX,BBY)(ipf;X(ix),Y(iy))=overlay measurement of printed field numberipf at intra-field position (X(ix),Y(iy))  (eq. 26)An average of the overlay measurements over printed fields may becalculated:(BBXavg,BBYavg)(X(ix),Y(iy))=average over printed fields (ipf) of(BBX,BBY)(ipf;X(ix),Y(iy))  (eq. 27)The intra-field distortion map can then be determined from theseaveraged measurements:(TX _(—) if,Ty _(—) if)(X(ix),Y(iy))=(BBXavg,BBYavg)(X(ix),Y(iy))  (eq.28)

First Exposure Averaging Embodiment

FIG. 38 is a flow diagram illustrating an improved technique fordetermining intra-field distortion. Techniques illustrated in FIG. 38reduce the effects of random error in the stepped reference pattern andprovide the ability to measure the average intra-field scan distortionwith fewer measurements. Flow begins in block 3802 where a reticle isprovided. The reticle may be loaded onto a reticle stage of a projectionimaging tool, or machine. Flow then continues to block 3804 and a photoresist coated wafer is provided. The wafer may be loaded onto a waferstage of a projection imaging tool. Flow continues to block 3806.

In block 3806, image field blades in the projection imaging system areapertured down on the reticle to an area corresponding to the projectedfield of the projection imaging tool. For example, a typical projectionimaging tool may be a 5× reduction stepper. In this example, the bladedarea on the reticle would correspond to a 22×22 mm² projected field onwafer W while in a typical 4× reduction scanner, it would be a 26×32 mm²projected field. FIG. 39 is a block diagram of a reticle illustratingthe bladed areas on a reticle. As shown in FIG. 39, a reticle R includesan array of alignment attributes AA and a single complementary alignmentattribute AA′. Field blades may be used to reduce the aperture, referredto as alignment attribute bladed area AABA, so that only the alignmentattributes will be exposed. Also shown in FIG. 39 is the use of fieldblades to reduce the aperture, referred to as complementary alignmentattribute bladed area AA′BA, so that only the complementary alignmentattribute will be exposed.

Returning to FIG. 38, in block 3806 field blades are positioned suchthat the alignment attribute bladed area is exposed with a single passat a dose greater than what is referred to as E0 (typicallyapproximately 2*E0). “E0” is the dose (in millijoules/cm²) on the waferrequired for the subsequent development step to clear a large feature,much greater than the resolution of the projection imaging tool, on thewafer. After exposure, the alignment attributes, AA, of the reticle Rwill have been printed on the wafer W, thereby forming printed alignmentattributes, PAA, of a single printed field, such as PF1 in FIG. 37.

Flow then continues to block 3808. In block 3808, image field blades arenow apertured down so that only the complementary alignment attribute,AA′, on reticle R will be exposed (AA′BA in FIG. 39). The complementaryalignment attribute is exposed in accordance with the operationsdepicted in FIG. 40.

FIG. 40 is a flow diagram illustrating a technique where a series of subnominal, or sub E0, exposures are used to fully expose a complementaryalignment attribute. It is a further breakdown of block 3808. Flowbegins in block 4002. In block 4002, a nominal dose (E_(nom)approximately equal to 2*E0) that is required to fully expose a reticlepattern onto a photo resist coated wafer is reduced, for example dividedinto a reasonably large (N larger than about 10) number of subexposures, to arrive at the exposure dose for the complementaryalignment attribute AA′, such that

$E_{sub} = {\frac{E_{nom}}{N}.}$Flow continues to block 4004 where the complementary alignmentattribute, area AA′BA in FIG. 39, is stepped over each PAA in a printedfield, such as PF1 in FIG. 37, and exposed with dose Esub. Step 4004 isrepeated until the complementary alignment attribute is fully exposed ateach of the PAAs. For example, each PAA in FIG. 37 includes an NX by NYarray of alignment attributes exposed. In the example of FIG. 37,ix=1:NX=1:3 and iy=1:NY=1:3 resulting in an array of 9 PAA within eachprinted field. Each of the PAAs is stepped over N times so that thetotal dose used to expose the complementary attribute equals a nominaldose. For example, if N=10 then the process in block 4004, where thecomplementary alignment attribute is exposed onto each PAA within theprinted field, is repeated 10 times. In other words, if N=10 and thereare 9 PPAs, the complementary alignment attribute would be exposed atotal of 90 times. Flow continues to block 4006 and the exposure iscomplete.

The process described in FIG. 40 results is a printed field (PF1)containing an array of printed completed alignment attributes (PCAA).Using N separate sub E0 exposure steps to fully expose the complementaryalignment attribute tends to average out the effects of wafer stage gridand yaw non-repeatabilites. In addition, the advantage of this averagingeffect is embodied in a single printed field (PF1) thereby eliminatingthe need to measure multiple printed fields and then average theresulting measurements. The sub E0 dose may be achieved in differentways. For example, a reduced transmission reticle may be used, or atransmission reducing element may be located within the optics of theprojection imaging system, or other techniques.

Returning to FIG. 38, after the repeated sub E0 exposures, flowcontinues to block 3810. In block 3810, the exposed wafer is developedand the overlay targets in printed field PF1 are read, or measured, inan overlay metrology tool.

Flow continues to block 3812. In block 3812 the projection imaging tool,or machine, intra-field distortion is reconstructed. The overlaymeasurements can be equated with the intrafield machine distortion as:(Tx _(—) if,Ty _(—) if)(X(ix),Y(iy))=(BBX,BBY)(ipf=1;X(ix),Y(iy))  (eq.29)

Comparing Equations 28 and 29 shows that use of the techniquesillustrated in FIG. 38 have determined the intrafield distortion mapwithout the need of averaging overlay measurements over multiple printedfields. An advantage of this technique of using N sub exposures to fullyexpose the complementary alignment attribute is that it has resulted ina decreased number of overlay measurements required to achieve the samenoise level by a substantial factor of

$\sim {N*\frac{E\; 0}{E_{nom}}} > \sim 5.$

Second Exposure Averaging Embodiment

FIG. 41 is a flow diagram illustrating another embodiment of techniquesfor reducing noise in measurements for determining intra fielddistortion. This technique applies to step and scan systems (scanners).Flow begins in block 4102 where a reticle is provided similarly to block3802 in FIG. 38. Flow continues to block 4104 where a wafer is providedsimilarly to block 3804 in FIG. 38. Flow continues to block 4106.

In block 4106, image field blades in the scanner are apertured down onreticle R to an area corresponding to the projected field of theprojection imaging tool, similarly to the techniques described above fora scanner. In block 4106 the entire scanner field is exposed usingmultiple sub nominal, or sub E0 exposures. Details of block 4106 areillustrated in FIG. 42.

FIG. 42 is a flow diagram illustrating techniques for using multiple subE0 doses to expose the entire scanner field. Flow begins in block 4202.In block 4202 a nominal dose (E_(nom) approximately equal to 2*E0) thatis required to fully expose a reticle pattern onto a photo resist coatedwafer is reduced, for example divided into a reasonably large (N′ largerthan about 10) number of sub exposures, to arrive at the exposure dosefor the alignment attributes AA, such that

$E_{sub}^{\prime} = {\frac{E_{nom}}{N^{\prime}}.}$Flow continues to block 4204 where the alignment attribute bladed areaAABA (see FIG. 39) is dynamically scanned N′ times at dose E′_(sub). Anadvantage of using multiple exposures is that it tends to average outthe scanning synchronization error so that subsequent overlaymeasurements reflect the repeatable part of the scanner synchronizationerror as much as possible. Flow continues to block 4206 where thealignment attributes, AA, of the reticle R have been printed on thewafer W thereby forming the printed alignment attributes, PAA, of asingle printed field (see PF1 in FIG. 37).

Returning to FIG. 41, flow continues to block 4108. In block 4108, thecomplementary alignment attribute is exposed using multiple sub E0exposures as described in the discussion of FIG. 38, block 3808 and FIG.40. Flow then continues to block 4110. In block 4110, the exposed waferis developed and the overlay targets in printed field PF1 are read, ormeasured, in an overlay metrology tool. Flow continues to block 4112 andthe projection imaging tool, or machine, intra-field distortion isreconstructed.

Third Exposure Averaging Embodiment

FIG. 43 is a flowchart illustrating another technique for reducing noisein measurements used for determining intra field distortion. Flow beginsin block 4302 where a reticle is provided. In this embodiment thereticle is configured so that the transmission of the complementaryattribute is reduced compared to a standard binary, such as chrome onglass, reticle. For example, light throughput through the portion of thereticle that has not had its transmission reduced may be characterizedby a transmissivity of T_(nom)=92% and the portion to the reticle thathas its transmission reduced may be characterized by a transmissivity ofT′_(nom)=6%. In this example, the portion of the reticle correspondingto the alignment attributes may have a transmissivity of T′_(nom)=92%and the portion of the reticle corresponding to the complementaryalignment attribute may have a transmissivity of T_(nom)=6%. FIG. 44 isa schematic diagram illustrating alignment attributes AA that are openbars in chrome. In FIG. 44, where the chrome has been removed, indicatedby the darkened region, there is clear glass so the transmissivity inthis region would be a high value, such as 92%. FIG. 45 is a schematicdiagram illustrating an example of a complementary alignment attribute.In FIG. 45, where the chrome has been removed, indicated by the darkenedregion, an attenuated phase shift mask material, for example having anominal transmissivity of 6%, is installed. Example of attenuated phaseshift mask material are known to those of skill in the art. See TheAttenuated Phase Shift Mask, B. Lin, supra. Other examples of amechanism for reducing the transmission of a region of the reticleinclude a dielectric, partially reflecting layer located on the reticleface or backside, as illustrated in FIG. 28. In this embodiment, thereduced transmission mechanism is localized to complementary alignmentattribute region of the reticle. In addition, instead of a partiallyreflecting backside layer, a partially absorbing one could be used.Also, in reflective reticles as shown in FIG. 30, such as used in EUV, acomplementary alignment attribute with reduced reflection could beprovided.

Returning to FIG. 43, after the reticle has been provided in block 4302,flow continues to block 4304 and a photo resist coated wafer isprovided. Flow continues to block 4306. In block 4306, image fieldblades in the projection imaging system are apertured down on thereticle to an area corresponding to the projected field of theprojection imaging tool and the reticle is exposed on the wafer. Flowcontinues to block 4308.

In block 4308 the image blades are apertured down on the reticle to anarea corresponding to the complementary alignment attribute. Because thetransmissivity of the portion of the reticle corresponding to thecomplementary alignment attribute has been reduced the exposure doserequired to fully develop the photo resist and create a printedcomplementary alignment attribute is greater by an amount correspondingto

$\frac{T_{nom}}{TCAA}$where TCAA is the transmission of the complementary alignment attribute.Thus, the reference exposure dose (Esub as described in block 4002 inFIG. 40 above) required to expose the printed complementary alignmentattribute to a level of Enom is:

$\begin{matrix}{E_{sub} = {E_{nom}*\frac{T_{nom}}{\left( {N*{TCAA}} \right)}}} & \left( {{eq}.\mspace{11mu} 30} \right)\end{matrix}$

Typically, dose E_(sub) is approximately the same as dose E0 and istherefore more readily accessible within the operational framework ofthe projection imaging tool. For example, if E_(nom)=2*E0, T_(nom)=0.92,TCAA=0.06, and N=20 exposures then, from Equation 30, E_(sub)˜1.5E0.This is contrasted with other embodiments which do not utilize adecreased transmission reticle where E_(sub)˜0.1E0. While this latterexposure dose of about 0.1E0 may be achievable on some projectionimaging tools, the former dose of about 1.5E0, which is approximately 15times greater, will typically be easier to achieve in practice. Anotheradvantage of this technique is the ability to average over a greaternumber, N, of instances of the reference complementary alignmentattribute placement.

Returning to FIG. 43, flow continues to block 4310. In block 4310, theexposed wafer is developed and the overlay targets are read, ormeasured, in an overlay metrology tool. Flow then continues to block4312 where the projection imaging tool, or machine, intra-fielddistortion is reconstructed as described above.

Fourth Exposure Averaging Embodiment

FIG. 46 is a flowchart illustrating another technique for reducing noisein measurements used for determining intra field distortion in ascanner. Flow begins in block 4602 where a reticle is provided. In thisembodiment, the portion of the reticle corresponding to both thealignment attributes and the complementary alignment attributes are bothconfigured to have reduced transmission. For example, the reducedtransmissivity of the alignment attributes and the complementaryalignment attributes may be reduced by the same amount, or they may bereduced by different amounts. Also, different mechanisms may be used toreduce the transmission of different portions of the reticle. Forexample, one mechanism may be used to reduce the transmission of theportion of the reticle corresponding to the alignment attributes, and adifferent mechanism may be used to reduce the transmission of theportion of the reticle corresponding to the complementary alignmentattribute. Likewise, different mechanisms may be used to reduce thetransmission of some of the alignment attributes and a differentmechanism may be used to reduce the transmission of other alignmentattributes on the same reticle. This is desirable when it is wished tohave different amounts of averaging (N=5 and N=20 say) for the sameE_(sub).

Flow continues to block 4604 where a wafer is provided. Flow thencontinues to block 4606. In block 4606 multiple dynamic exposures of thescanner field are performed. Multiple exposures of the scanner fieldhelps to reduce the effect of scan synchronization error onmeasurements. Using the reduced transmission reticle, the per scan doseis generally more accessible within the operational framework of atypical scanner. Flow continues to block 4608.

In block 4608 the image blades are apertured down on the reticle to anarea corresponding to the complementary alignment attribute. Asdescribed above, for block 3808 in FIG. 38, the transmission of theportion of the reticle corresponding to the complementary alignmentattribute has been reduced, causing the exposure dose required to fullydevelop the photo resist and create a printed complementary alignmentattribute to be greater by an amount corresponding to

$\frac{T_{nom}}{TCAA}.$Flow continues to block 4610 and the exposed wafer is developed and theoverlay targets are read, or measured, in an overlay metrology tool.Flow then continues to block 4612 where the projection imaging tool, ormachine, intra-field distortion is reconstructed as described above.

Other Embodiments and Variations

FIG. 47 is a block diagram of an example of a projection imaging tool(PIT). As shown in FIG. 47, the projection imaging tool includes aneffective source ES, a reticle stage RS, projection imaging optics PIO,and a wafer stage WS. The effective source ES includes a light sourceLS, input illuminator optics IIO and output illumination optics OIO.

The reticle stage RS holds a pellicle PE reticle (R) combination. Forexample, the reticle stage may be used to hold, and position, reticlesconfigured as described in the above embodiments.

The projection imaging optics include input projection optics, anaperture stop, and output projection optics. The wafer stage WS isconfigured to hold and position a photo resist coated wafer.

Heretofore, it has been considered the reticle creating the overlaypatterns as perfect. In practice it is not, but errors in the reticlemanufacture can be taken into account by first measuring the position ofall the individual structures in all of the overlay groups using anabsolute metrology tool such as the Nikon 5I (See Measuring SystemXY-5i, supra), or Leica LMS 3200 series tools. Next, in formulatingEquations 20-23, this reticle error (divided by the photolithographicexposure tool demagnification) is explicitly written out on the righthand side and then subtracted from the resulting overlay measurements onthe left hand side of the equations (thereby canceling out on the righthand side). The result is Equations 20-23 as they are written above butwith a correction applied to the overlay measurements appearing on theleft hand side. The analysis then proceeds word for word as before.

The reticle of the present invention is typically glass or fused silicawith openings defined in a chrome coating. This is common for projectionlithography tools utilized in semiconductor manufacture. The form thereticle can take will be determined by the format required by thespecific projection imaging tool on which the reticle is loaded. Thusfor purposes of analyzing copying machine performance, the reticle OL ofthe present invention would consist of a piece of paper or mylar withoverlay groups disposed on it. In an extreme ultra violet (EUV) exposuretool the mask would be reflective.

The completed alignment attributes of the present invention so fardiscussed are of the box in box, bar in bar, or wafer alignment marksmost commonly used in semiconductor manufacture. In practice, hundredsof different overlay target patterns are available (See Handbook ofMicrolithography and Microfabrication, supra; Direct-ReferencingAutomatic Two-Points Reticle-to-Wafer Alignment Using a ProjectionColumn Servo System, M. Van den Brink et al., SPIE Vol. 633, OpticalMicrolithography V, 60:71, 1986; Overlay Alignment Measurement ofWafers, N. Bareket, U.S. Pat. No. 6,079,256, Jun. 27, 2000; FIG. 1),some common completed alignment attributes are shown in FIG. 3. Theexact form taken by the completed alignment attributes will bedetermined by the overlay metrology used in the measurement step.

The overlay metrology tool utilized by the present invention istypically a conventional optical overlay tool such as those manufacturedby KLA-Tencor (See KLA 5105 Overlay Brochure, supra; KLA 5200 OverlayBrochure, KLA-Tencor) or Bio-Rad Semiconductor Systems. See Quaestor Q7Brochure, Bio-rad Semiconductor Systems. Other optical overlay toolsthat can be used by the present invention include those described in SeeProcess for Measuring Overlay Misregistration During Semiconductor WaferFabrication, I. Mazor et al., U.S. Pat. No. 5,438,413, Aug. 1, 1995. Inaddition, some steppers or scanners (See Matching Management of MultipleWafer Steppers Using a Stable Standard and a Matching Simulator, supra)can utilize their wafer alignment systems and wafer stages to functionas overlay tools. However, in this role we would limit the total size ofthe alignment attribute (consisting of 2 wafer alignment marks) to adistance over which the wafer stage would be as accurate as aconventional optical overlay tool. This distance is typically less thanabout 2.0 mm. When electrical alignment attributes are used for overlay(See Matching Management of Multiple Wafer Steppers Using a StableStandard and a Matching Simulator, supra; Automated ElectricalMeasurements of Registration Errors in Step and Repeat OpticalLithography Systems, T. Hasan et al., IEEE Transaction on ElectronDevices, Vol. ED-27, No. 12, 2304:2312, December 1980; Capacitor CircuitStructure for Determining Overlay Error, K. Tzeng et al., U.S. Pat. No.6,143,621, Nov. 7, 2000), the overlay metrology tool as utilized by thisinvention would correspond to the electrical equipment utilized formaking the corresponding measurement.

The present invention has been mainly described with respect to itsapplication on the projection imaging tools (scanners (See Micrascan™III Performance of a Third Generation, Catadioptric Step and ScanLithographic Tool, D. Cote et al., SPIE Vol. 3051, 806:816, 1997; ArFStep and Scan Exposure System for 0.15 Micron and 0.13 Micron TechnologyNode, J. Mulkens et al., SPIE Conference on Optical MicrolithographyXII, 506:521, March 1999; 0.7 NA DUV Step and Scan System for 150 nmImaging with Improved Overlay, J. V. Schoot, SPIE Vol. 3679, 448:463,1999) commonly used in semiconductor manufacturing today. The methods ofthe present invention can be applied to other scanning projection toolssuch as; 2-dimensional scanners (See Large-Area, High-Throughput, HighResolution Projection Imaging System, Jain, U.S. Pat. No. 5,285,236,Feb. 8, 1994; Optical Lithography—Thirty Years and Three Orders ofMagnitude, supra), office copy machines, and next generation lithography(ngl) systems such as XUV (See Development of XUV Projection Lithographyat 60-80 nm, B. Newnam et al., SPIE Vol. 1671, 419:436, 1992), SCALPEL,EUV (Extreme Ultra Violet) (See Reduction Imaging at 14 nm UsingMultilayer-Coated Optics: Printing of Features Smaller than 0.1 Micron,J. Bjorkholm et al., Journal Vacuum Science and Technology, B 8(6),1509:1513, November/December 1990), IPL (Ion Projection Lithography),and EPL (electron projection lithography). See Mix-and Match: ANecessary Choice, supra.

The present invention has been mainly described with respect to therecording medium being positive photoresist. The present invention couldequally well have used negative photoresist providing we makeappropriate adjustment to the overlay groups on the reticle. In general,the recording medium is whatever is typically used on the lithographicprojection tool we are measuring. Thus, on an EPL tool, an electron beamphotoresist such as PMMA could be utilized as the recording medium.

So far, we have described the substrates on which the recording media isplaced as wafers. This will be the case in semiconductor manufacture.The exact form of the substrate will be dictated by the projectionlithography tool and its use in a specific manufacturing environment.Thus, in a flat panel manufacturing facility, the substrate on which thephotoresist would be placed would be a glass plate or panel. A maskmaking tool would utilize a reticle as a substrate. Circuit boards ormulti-chip module carriers are other possible substrates.

The foregoing description details certain embodiments of the invention.It will be appreciated, however, that no matter how detailed theforegoing appears, the invention may be embodied in other specific formswithout departing from its spirit or essential characteristics. Thedescribed embodiments are to be considered in all respects only asillustrative and not restrictive and the scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription. All changes, which come with the meaning and range ofequivalency of the claims, are to be embraced within their scope.

1. A reticle for use in determining lens distortion in a projectionimaging tool, the reticle comprising: at least one alignment attribute;and a complementary alignment attribute, wherein the transmissivity ofthe alignment attribute is different than the transmissivity of thecomplementary alignment attribute.
 2. A reticle as defined in claim 1,wherein the transmissivity of the alignment attribute is greater thanthe transmissivity of the complementary alignment attribute.
 3. Areticle as defined in claim 1, wherein, the transmissivity of thealignment attribute is less than the transmissivity of the complementaryalignment attribute.
 4. A reticle as defined in claim 1, wherein the atleast one alignment attributes comprise an array of alignmentattributes.
 5. A reticle as defined in claim 4, wherein thetransmissivity of some of the alignment attributes is different than thetransmissivity of others of the alignment attributes.
 6. A reticle asdefined in claim 1, further comprising phase shift mask material.
 7. Areticle as defined in claim 1, further comprising reflective material.8. A reticle as defined in claim 1, further comprising anti-reflectivematerial.